In conventional practice, undesirable thermal stress develops because of differences in the coefficient of thermal expansion between the mounting board and semiconductor element of flip-chip semiconductor device or when surface-mounted electronic components are mounted on mounting boards such as chip size packages (CSP). Specifically, the drawback is that when a semiconductor element is mounted on a mounting board, cracking is induced in the semiconductor element or mounting board by the thermal stress generated between the semiconductor element and the mounting board, or peeling occurs in the soldered joints between the mounting board and the semiconductor element.
To overcome this drawback, methods have been developed in which a buffer portion for reducing the thermal stress is provided on the side of the package on which the semiconductor element is mounted, and excessive stress is prevented from developing in the semiconductor element. Examples of such methods include those in which a semiconductor element is mounted on a board via a buffer layer formed from an elastomer or other buffer material in the form of a film (for example, JP (Kokai) 4-363032, 10-64927, and 2001-298272); and those in which connection terminals connected to the electrodes of semiconductor elements are formed from wires to render the connection terminals elastic and to allow them to be deformed. There are also methods in which joints between the semiconductor elements and multilayer board are packed with an underfiller to fix the semiconductor elements in place following mounting.
Some substrate boards are fabricated by forming an insulating layer from resin alone, but such resin films, in addition to problems associated with insulation properties, heat resistance, and peel strength characteristics, also have problems associated with film cracking during film forming or film winding, particularly in the case of epoxy resins. It is known that resin cracking and other such drawbacks can commonly be overcome if acrylonitrile-butadiene rubber is added to the resin component to reduce its modulus of elasticity, but because problems related to moisture resistance and heat resistance are encountered when the rubber is added in a large amount, it is impossible to obtain viable substrate boards merely by adding rubber to resin. It can also be indicated that because a film composed of resin alone does not have a base material, the flowability of the resin is difficult to control, and problems are encountered in terms of thickness accuracy and insulation properties during lamination and processing.
Because of considerations related to impedance control and the like, the thickness of the insulating material in current substrate boards (multilayer printed boards) must be set to a value designed by computer simulation. High accuracy is therefore needed in terms of uniform thickness, composition, and the like. Currently there are products in which a polyimide, a film composed of resin alone, or the like is used as the core layer, and this layer is coated on both sides with a resin (JP (Kokai) 2000-144072), but it has been pointed out that these products fail to satisfy high reliability attributes such as material uniformity and resin flowability.
Thus, all the printed wiring boards designed in accordance with the prior art have drawbacks, and measures designed to ensure the desired level of reliability in solder joints are becoming increasingly more vital as semiconductor chips are bonded with solder over smaller areas in response to the miniaturization of semiconductor devices.
JP (Kokai) 9-46012 discloses, as an item related to the present invention, a flexible wiring board obtained using an adhesive layer with a low modulus of elasticity, itself obtained by curing a thermoplastic porous resin film after it has been impregnated with a thermosetting resin composition that has a low modulus of elasticity. In this invention, however, a low-elasticity adhesive layer is used in order to prevent flexible boards from warping, and this layer cannot be utilized to prevent cracking from developing between a rigid board and a semiconductor element when this element is mounted on the board in the same manner as in the present invention.
An object of the present invention is to provide a stress-relief dielectric film in which drawn porous polytetrafluoroethylene whose structure remains flexible across a variety of temperature regions is used as a base material, whereby the dielectric film is provided with characteristics such as reduced modulus of elasticity and increased elongation; solder can be directly used to form joints when employed to connect boards and semiconductor elements normally affected by expansion and contraction caused by thermal stress, metal fatigue caused by such repeated expansion and contraction, and other problems; the semiconductor elements, solder joints, and boards are prevented from cracking even without the use of a conventionally employed underfill material or other connection-forming material; the flexible characteristics of the board are utilized; post-pressing thickness accuracy can be ensured together with adequate embedding properties that allow internal-layer circuitry to be firmly held in place; excellent insulation properties, adhesion, and reliability are achieved; handling is improved during film winding and processing; and higher productivity can be ensured; and also to provide a printed wiring board and a semiconductor device in which this film is used.